This invention relates to a semiconductor non-volatile memory element of an electrically erasable type, in which the number of possible write/erase cycles is increased.
A typical conventional electrically erasable read only memory (EEPROM) has a two-layer gate structure including a floating gate made of polycrystalline silicon (poly-Si) formed through a first insulating layer on a semiconductor substrate and a control gate formed through a second insulating layer on the floating gate. See, for example "Electron Trapping in Oxide Grown from Polycrystalline Silicon", by Chenming Hu, D. Y. Joh, Y. Shum, and T. Klein, disclosed in CH1504-0/79/0000-0229500.75 .COPYRGT.1979 IEEE, PP 229 through 231. As mentioned in the above reference, writing is achieved by depositing electrons on the floating gate through avalanche or channel-hot electron injection so as to raise the threshold voltage Vt of the transistor. Erasing is effected by removing the electrons from the floating gate across the second insulating layer to the control gate, with the removal rate limited by the Fowler-Nordhein tunnel emission at the poly-Si/SiO.sub.2 interface.
The problem in the conventional EEPROM is that the EEPROM becomes useless after a certain limited number of write/erase cycles, because the threshold voltage Vt rises after erasing. It is believed that the fatigue is caused by electron trapping in the second insulating layer formed on the poly-Si floating gate. As the write/erase cycle increases, the density of trapped charge increases causing the electric field at the poly-Si/SiO.sub.2 interface to decrease, whereby erasing becomes progressively more difficult.
In the conventional EEPROM, the second insulating layer formed between the poly-Si floating gate and the control gate has a thickness of about 800 through 1500 angstroms formed by thermal oxidation of poly-Si at a temperature of about 800.degree. C. through 1100.degree. C. From here on, the oxide film formed by thermal oxidation of poly-Si is referred to as poly-OX. Such a thick poly-OX formed at such a low temperature causes a great number of electron traps described in detail hereinafter. Therefore, the conventional EEPROM becomes useless only after 100 to 1000 write/erase cycles and cannot be commercially used.